PCB Tech - PCB cooling and IC package heat dissipation

The IC package relies on the PCB to dissipate heat. Generally speaking, PCB is the main cooling method for high power consumption semiconductor devices. A good PCB heat dissipation design has a huge impact. It can make the system work well, and it can also bury the hidden dangers of thermal accidents. Careful handling of PCB layout, board structure, and device placement can help improve the thermal performance of mid-to-high power applications.

It is difficult for semiconductor manufacturing companies to control the systems that use their devices. However, the system in which the IC is installed is critical to overall device performance. For custom IC devices, system designers usually work closely with manufacturers to ensure that the system meets the many heat dissipation requirements of high-power devices. This early collaboration can ensure that the IC meets electrical standards and performance standards, while ensuring normal operation in the customer's cooling system. Many large semiconductor companies sell devices as standard parts, and there is no contact between the manufacturer and the end application. In this case, we can only use some general guidelines to help achieve a better IC and system passive cooling solution.

The common semiconductor package type is exposed pad or PowerPADTM package. In these packages, the chip is mounted on a metal sheet called a die pad. This chip pad supports the chip during chip processing and is also a good thermal path for heat dissipation of the device. When the exposed pad of the package is soldered to the PCB, the heat can quickly dissipate from the package and then enter the PCB. After that, the heat is dissipated through each PCB layer and into the surrounding air. Exposed pad packages generally conduct about 80% of the heat, which enters the PCB through the bottom of the package. The remaining 20% of the heat is dissipated through the device wires and all sides of the package. Less than 1% of the heat is dissipated through the top of the package. For these exposed pad packages, a good PCB heat dissipation design is essential to ensure certain device performance.

The first aspect of PCB design that can improve thermal performance is the layout of PCB components. Whenever possible, high-power components on the PCB should be separated from each other. This physical separation between high-power components maximizes the PCB area around each high-power component, thereby helping to achieve better heat conduction. Care should be taken to isolate temperature-sensitive components on the PCB from high-power components. Whenever possible, the installation location of high-power components should be far away from the corners of the PCB. A more central PCB location can maximize the board area around high-power components, thereby helping to dissipate heat. Figure 2 shows two identical semiconductor devices: components A and B. Component A is located at the corner of the PCB and has a chip junction temperature that is 5% higher than that of component B, because component B is located closer to the middle. Since the board area around the component for heat dissipation is smaller, the heat dissipation at the corner of component A is limited.

The second aspect is the structure of the PCB, which has the most decisive influence on the thermal performance of the PCB design. The general principle is: the more copper in the PCB, the higher the thermal performance of the system components. The ideal heat dissipation situation for semiconductor devices is that the chip is mounted on a large piece of liquid-cooled copper. For most applications, this mounting method is impractical, so we can only make some other changes to the PCB to improve the heat dissipation performance. For most applications today, the total volume of the system continues to shrink, which has an adverse effect on heat dissipation performance. The larger the PCB, the larger the area that can be used for heat conduction, and it also has greater flexibility, allowing enough space between the high-power components.

Maximize the number and thickness of PCB copper ground planes whenever possible. The weight of the ground layer copper is generally relatively large, and it is an excellent thermal path for the entire PCB to dissipate heat. The arrangement of wiring for each layer will also increase the total proportion of copper used for heat conduction. However, this wiring is usually electrically and thermally isolated, which limits its role as a potential heat dissipation layer. The wiring of the device ground plane should be as electrical as possible with many ground planes, so as to help maximize heat conduction. The heat dissipation vias on the PCB under the semiconductor device help heat to enter the buried layers of the PCB and conduct to the back of the circuit board.

For improving heat dissipation performance, the top and bottom layers of PCB are "golden locations". Use wider wires and route them away from high-power devices to provide a thermal path for heat dissipation. The dedicated thermal board is an excellent method for PCB heat dissipation. The thermal board is generally located on the top or back of the PCB, and is thermally connected to the device through direct copper connections or thermal vias. In the case of inline packaging (packages with leads on both sides only), this kind of thermal conductive plate can be located on the top of the PCB and shaped like a "dog bone" (the middle is as narrow as the package, and the area away from the package is relatively small. Large, small in the middle and large at the ends). In the case of a four-side package (with leads on all four sides), the heat-conducting plate must be located on the back of the PCB or enter the PCB.

Increasing the size of the thermal board is an excellent way to improve the thermal performance of the PowerPAD package. Different thermal board sizes have a great influence on thermal performance. The product data sheet provided in the form of a table generally lists these size information. However, it is difficult to quantify the impact of the added copper of custom PCBs. Using some online calculators, users can select a device and then change the size of the copper pad to estimate its impact on the heat dissipation performance of non-JEDEC PCBs. These calculation tools highlight the impact of PCB design on thermal performance. For a four-side package, the area of the top pad is just smaller than the area of the exposed pad of the device. In this case, the buried or back layer is the first way to achieve better cooling. For dual in-line packages, we can use a "dog bone" pad style to dissipate heat.

Finally, a larger PCB system can also be used for cooling. In the case that the screws are connected to the heat-conducting plate and ground plane for heat dissipation, some screws used to mount the PCB can also become effective heat paths to the system base. Considering the heat conduction effect and cost, the number of screws should be the maximum value that reaches the point of diminishing returns. After being connected to the thermal conductive plate, the metal PCB reinforcement plate has more cooling area. For some applications where the PCB is covered with a shell, the type controlled welding repair material has a higher thermal performance than the air-cooled shell. Cooling solutions, such as fans and heat sinks, are also common methods for system cooling, but they usually require more space or need to modify the design to optimize the cooling effect.